mimeTypeApplies example

  public static function isApplicable(FieldDefinitionInterface $field_definition) {
    if (!parent::isApplicable($field_definition)) {
      return FALSE;
    }
    /** @var \Symfony\Component\Mime\MimeTypeGuesserInterface $extension_mime_type_guesser */
    $extension_mime_type_guesser = \Drupal::service('file.mime_type.guesser.extension');
    $extension_list = array_filter(preg_split('/\s+/', $field_definition->getSetting('file_extensions')));

    foreach ($extension_list as $extension) {
      $mime_type = $extension_mime_type_guesser->guessMimeType('fakedFile.' . $extension);
      if (static::mimeTypeApplies($mime_type)) {
        return TRUE;
      }
    }
    return FALSE;
  }

  /** * {@inheritdoc} */
  public function settingsSummary() {
    $summary = [];